Olson, U.S. Pat. No. 2,964,272 discloses the basic concept behind vibration cancellation systems. Olson discloses a system comprised of a driving element, a vibration sensing element and an electrical signal amplifier. The sensing element senses the vibration of a structure and converts the vibration to an electrical signal which is then sent to the amplifier. The amplifier amplifies this signal which operates the driver. The driver, which is mounted between the vibrating structure and a static structure, converts the signal from the amplifier into a mechanical force. By adjusting the phase of the signal coming from the amplifier the driver can be operated so as to counteract the vibration emanating from the structure.
Over the years various forms and types of vibration cancellation systems have been suggested and employed in a variety of applications.
Boothe, U.S. Pat. No. 3,189,303 discloses an active mount for supporting heavy machinery comprised of a pneumatic container or air spring for supporting a load, and a system for supplying air to the container in accordance with the force applied thereto by the load. The pneumatic system utilizes an air source for supplying air to a first valve which is positioned to be actuatable by the force of the load and designed to produce a pressure which is proportional to the force of the load. Air from the first regulator is directed through a stabilizing zone to a second valve and from the second valve, the air is delivered to the air spring.
Curwen, U.S. Pat. No. 3,216,679 discloses an active vibration isolator comprised of a vertically displaceable piston disposed within a cylinder and having means for receiving a load. A valve controls the flow of gas into the cylinder so as to move the piston vertically in response to the load.
Scharton et al., U.S. Pat. No. 3,606,233 discloses a combined active and passive isolation mount wherein the active portion includes a piston, coupled to the vibration sources via a piston rod, adapted for movement within a cylinder to which the isolated mass is mechanically coupled. The piston separates two chambers within the cylinder. The system provides isolation by using a servovalve to control the relative pressure between the chambers in such a manner that the velocity of the cylinder counteracts the velocity of the piston.
Schubert et al., U.S. Pat. No. 3,701,499 discloses an active isolation system that includes a servovalve controlled hydraulic actuator to cancel vibrations
Malueg, U.S. Pat. No. 4,033,541 discloses a system that uses linear actuators to stabilize sensitive apparatus from translational and rotational vibrations emanating from the structure on which the apparatus is mounted.
Phillips, U.S. Pat. No. 4,336,917 discloses a shock and vibration isolation system having a plurality of isolators. Each isolator has two gas driven pistons connected to an accumulator/controller that supplies controlled amounts of air. The flow of gas into the accumulator/controller is governed by a valve.
Van Gerpen, U.S. Pat. No. 4,363,377 discloses an active seat suspension control system in which a hydraulic cylinder is coupled to the seat. A source of pressurized fluid to the cylinder allows the vertical position of the seat to be adjusted. The amount of gas flow into the cylinder is governed by an electro-hydraulic valve.
Abrams et al., U.S. Pat. No. 4,546,960 discloses a vibration isolation assembly which includes a servovalve in operative communication between a fluid supply source, a gas supply source and a viscous damper. Control logic means governs the servovalve to adjust both the gas and fluid pressures in the viscous damper in response to sensed vibration so as to attenuate the vibration.
Schubert, U.S. Pat. No. 4,757,980 discloses a vibration isolation system comprising a damper having a servovalve fluidly coupling a load supporting actuator to an accumulator.
Decker et al., U.S. Pat. No. 4,802,648 discloses an engine mount having an air cushion which functions like a pneumatic spring element and which can be inflated and vented via a valve device.
Hoying et al., U.S. Pat. No., 4,828,234 discloses a hydraulic mount assembly with a self-pumping air bladder. The pressurization of the bladder is controlled by a pneumatic control circuit that includes check valves and shuttle valves.
Each of the arrangements described in the above mentioned patents employs some type of mechanical device such as a valve, servovalve or actuator to transform the amplitude and frequency of an electrical cancelling signal into a mechanical cancelling oscillation having the same frequency and amplitude. Because these devices are comprised of mechanically linked, moving parts, they suffer from high distortion when subjected to high frequency signals, and consequently, are unable to vary the pressure of a gas or fluid fast enough to generate a consistent cancelling signal. Thus for example, the systems in Abrams et al., Scharton et al., Schubert et al., and Malueg all have an upper frequency limit of about 100 Hz.
Accordingly, a need exists for a vibration cancellation mount that can operate at frequencies substantially greater than 100 Hz without suffering from the high distortion experienced by the systems found in the prior art.